A number of loci which control radiation sensitivity in Neurospora crassa have been identified. The DNA repair processes have been characterized to some extent on the bases of responses of mutant strains to radiations and chemical agents. This project is an analysis of the interactions of mutant alleles that individually confer radiation sensitivity with regard to their effects on radiation-induced inactivation and forward-mutation induction. The lethal and mutagenic effects of UV and x-ray irradiation in these double-mutant strains are interpreted in terms of the repair systems in Neurospora.